The present invention relates to a method for determining the low pressure exhaust gas recirculation mass flow in the air system of an internal combustion engine.
Exhaust gas recirculation comprises an important measure for reducing the nitrogen oxide emissions which are produced upon the combustion of fuel in internal combustion engines. This measure enables the recirculation of a portion of the exhaust gas into the inlet space of the internal combustion engine. Production of nitrogen oxides during combustion is lessened by the recirculation of a portion of the exhaust gas which, being an inert gas, is nonreactive. Usually, the exhaust gas is recirculated into the inlet space by mixing in some of the exhaust gas of the inlet fresh air. An exhaust gas recirculation valve is provided for the metering of the exhaust gas. Also known in addition to a near engine high pressure exhaust gas recirculation system is a low pressure exhaust gas recirculation system in which, as a rule, the exhaust gas recirculation section is implemented downstream of a diesel particulate filter and upstream of a compressor.
Controlling the exhaust gas recirculation system requires the detection of various mass flows. The mass flow via the low pressure exhaust gas recirculation line is usually modeled, the low pressure exhaust gas recirculation mass flow being determined by means of a so-called throttle equation which involves the pressure upstream and the pressure downstream of the low pressure side exhaust gas recirculation valve, the temperature upstream of the low pressure side exhaust gas recirculation valve, and the geometry of the low pressure side exhaust gas recirculation valve. The problem arises in this case that generally only small pressure drops are present across the low pressure exhaust gas recirculation valve, and so this determination of the mass flow is strongly subject to tolerances, depending on the operating point.